void main(List<String> args) {
  num price = 20;
  print(num);

  int index = 10;
  print(index);

  double goods_price = 19.9;
  print(goods_price);

  //num不可以给int和double赋值

//int可以给num赋值
//double可以给num赋值
  price = goods_price;
  price = index;

  //不能全部用num声明 因为num会占用int和double的空间
}
